練習問題への回答

app.js

@@ -1 +1,45 @@
 'use strict';
+const fs = require('fs');
+const readline = require('readline');
+const rs = fs.ReadStream('./popu-pref.csv');
+const rl = readline.createInterface({ 'input': rs, 'output': {} });
+
+const map = new Map();
+rl.on('line', (lineString) => { // rlオブジェクトでlineイベントが発生した時の処理
+    const columns = lineString.split(',');
+    const year = parseInt(columns[0]);
+    const prefecture = columns[2];
+    const popu = parseInt(columns[7]);
+    if (year === 2010 || year === 2015) {
+        let value = map.get(prefecture);
+        if (!value) {
+            value = {
+                popu10: 0,
+                popu15: 0,
+                change: null
+            };
+        }
+        if (year === 2010) {
+            value.popu10 += popu;
+        }
+        if (year === 2015) {
+            value.popu15 += popu;
+        }
+        map.set(prefecture, value);
+    }
+});
+rl.resume();
+rl.on('close', () => {
+    for (let pair of map) {
+        const value = pair[1];
+        value.change = value.popu15 / value.popu10;
+    }
+    // mapを配列に変換後、sort関数で並び替え
+    const rankingArray = Array.from(map).sort((pair1, pair2) => {   // 戻り値が正の時に順番を入れ替える
+        return pair1[1].change - pair2[1].change;
+    })
+    const rankingStrings = rankingArray.map((pair, i) => {
+        return (i + 1) + "位:" + pair[0] + ': ' + pair[1].popu10 + '=>' + pair[1].popu15 + ' 変化率:' + pair[1].change;
+    })
+    console.log(rankingStrings);
+})